There are many applications for fumed silicas of extremely fine particle size in which it is convenient to apply the fumed silica in the form of an aqueous colloidal dispersion. Such applications include non-slip floor waxes, foamed rubber latices, paper coatings, the sol-gel process for the manufacture of optical fibers and quartz glassware, and thermal insulation. Aqueous colloidal dispersions of fumed silica are also utilized for frictionizing and polishing. There are also many occasions where it is convenient to densify fumed silica for storage or transport by combining the fumed silica with water to form an aqueous colloidal dispersion.
Fumed silica is generally produced by the vapor phase hydrolysis of chlorosilanes, such as silicon tetrachloride, in a hydrogen oxygen flame. The overall reaction is: EQU SiCl.sub.4 +2H.sub.2 +O.sub.2 .fwdarw.SiO.sub.2 +4 HCl
In this process, submicron sized molten spheres of silica are formed. These particles collide and fuse to form three dimensional, branched, chain-like aggregates, of approximately 0.1 to 0.5 microns in length. Cooling takes place very quickly, limiting the particle growth and insuring the fumed silica is amorphous. These aggregates in turn form agglomerates ranging in size from 0.5 to 44 microns (325 US mesh). Fumed silicas generally have very high purity, with total impurities, in many cases below 100 ppm (parts per million). This high purity makes fumed silica aqueous dispersions particularly advantageous for many applications.
Another consideration for many applications is the removal of grit from the aqueous dispersion of fumed silica since grit is a major source of impurities. Grit can also interfere with many applications of the dispersion. For example, in coagulation of latex rubber, grit will lead to the formation of defects in the structure of the rubber, and in the polishing of semiconductor single crystals grit can cause scratching. Thus it is generally desirable that the aqueous dispersion be of high purity. One method for increasing purity is to pass the aqueous colloidal dispersion through a filter, also referred to as filtering, to remove grit and other impurities. In order for an aqueous colloidal dispersion to be filterable, the viscosity of the colloidal dispersion must be low enough, and the colloidal dispersion must be non-dilatant to enable the colloidal dispersion to pass through the desired filter. For the purposes of the present invention, a non-dilatant dispersion is a dispersion which will pass through a filter having a pore size of 1000 microns or smaller.
As described above, the ability of a dispersion to pass through a filter is also related to the viscosity of a dispersion. The finer the filter, i.e. the smaller the size of the pores of the filter, the lower the viscosity of the aqueous colloidal dispersion must be to pass through the filter. As will be appreciated by those of ordinary skill in the art, to increase purity, the colloidal dispersion should be passed through as fine a filter as possible. Thus it is generally advantageous to produce aqueous colloidal dispersions with low viscosities. For the purposes of the present invention low viscosities are viscosities below about 1000 centipoise.
Additionally, in order to be useful for the applications listed above and other potential applications the aqueous colloidal dispersion cannot gel into a solid. The ability of the aqueous colloidal dispersion to resist gelling is generally referred to as the stability of the aqueous colloidal dispersion. More stable aqueous colloidal dispersions will not gel as soon as less stable aqueous colloidal dispersions.